If Angel of Serenity leaves the battlefield before its enters-the-battlefield ability has resolved, its leaves-the-battlefield ability will trigger and do nothing. Then the enters-the-battlefield ability will resolve and exile the targeted creatures and/or creature cards indefinitely.

1/24/2013

You choose up to three total targets from among other creatures on the battlefield and creature cards in graveyards. You may choose zero targets.

1/24/2013

You choose the targets as part of putting the enters-the-battlefield trigger on the stack. Because that ability includes “you may,” you choose whether to exile the targets when the ability resolves.

1/24/2013

If a creature targeted by the enters-the-battlefield ability dies before that ability resolves, it will become an illegal target even though it may be a creature card in a graveyard when the ability resolves. It won't be exiled.